This invention relates generally to improvements in magnetic bearing structures. More particularly, the present invention relates to a magnetic thrust bearing which utilizes a combination of controllable electromagnets and constant flux permanent magnets associated with a stationary member, to axially flux couple a rotatable member to the stationary member.
Electromagnetic bearings are highly effective for supporting a body, such as a rotating shaft, which is effectively floated or levitated by magnetic fields. In this way the rotating shaft has no frictional contact with any stationary structure, thereby permitting relatively friction free rotation of the shaft or rotation of a body about the shaft. This arrangement possesses the obvious advantage that there is no mechanical abrasion, which results in reduced mechanical noise and durability not available with other types of bearing structures. Moreover, because of the reduced frictional effects which would otherwise be encountered with conventional bearing structures, it is possible to obtain higher speeds of rotation with electromagnetic bearings.
Magnetic bearings typically require little maintenance and readily lend themselves to operation in hostile environments such as in connection with corrosive fluids where other conventional bearings would be destroyed or rendered inoperable. Further, magnetic bearings are suitable for supporting moving bodies in a vacuum, such as in outer space, or in canned pumps where the pump rotor must be supported without the use of physically contacting bearings.
One of the primary considerations in the development of magnetic bearing structures is to eliminate so-called air gaps. The so-called air gaps form a portion of the magnetic flux pathway of the electromagnets and permanent magnets, and provide a bridge between a supporting structure and a levitated structure. In actuality, some air gaps must be tolerated in order to position a suspended or rotatable body. Thus, air gaps to some extent cannot be avoided, but it is desirable to reduce air gaps to an absolute minimum.
From a pure physics standpoint, an air gap introduces great inefficiency into any type of magnetic structure. An air gap is about 2,000 times less efficient than an iron core medium for transmitting magnetic flux. Thus, in terms of inefficiency, a magnetic bearing structure which has an air gap of 0.1 inch is far more inefficient than a magnetic bearing which has an iron gap of 20 inches.
In addition, it is important to overcome the conductivity constraints of permanent magnets. Essentially, permanent magnets are very poor conductors for a magnetic flux, even though they generate magnetic flux. The most efficient permanent magnets available are the rare earth alloy magnets. Such permanent magnets, however, have a very low magnetic permeability and they behave in much the same manner as air gaps in the magnetic circuit. The low permeability of rare earth alloy magnets requires significant power to drive electromagnetic fields through the permanent magnets, thereby resulting in low electrical efficiencies. Thus, it is undesirable to transmit an electromagnetic field through a permanent magnet.
Early magnetic thrust bearings consisted of two solenoids positioned on each side of a rotatable disc which is an integral part of the supported shaft. Such early designs utilized the electromagnetic solenoid coils to create a magnetic field in the two air gaps between the solenoids and the rotatable disc. A position sensor measured the disc axial location and a closed loop servo system maintained the shaft in the desired axial location. The entire magnetic field between the disc and the solenoids was generated by electric currents in the solenoid coils. Further, a large bias current was normally applied to the coils to generate a magnetic field and to linearize the relationship between input current and force produced on the disc. This method simplified the design of the servo controls.
There were several disadvantages of this early approach. First, very large electromagnet coils were required because of the requirement to generate and maintain continuously the entire magnetic field in the air gaps. Secondly, large amounts of input electrical power were required to maintain the magnetic fields, which resulted in a large electronics control system. Thirdly, two power amplifiers were required to develop bi-directional forces since the electromagnets were only capable of producing attraction forces.
Many of these disadvantages have been overcome by incorporating permanent magnets into thrust bearings. Newer types of magnetic thrust bearings utilize a radially polarized permanent magnet disc which forms an integral part of the rotatable disc. Two solenoids disposed about the thrust disc in facing relation on opposite sides thereof are utilized as before. This improved design has the advantages of linearizing the force versus solenoid control current, greatly reducing the required electrical power to develop forces on the disc, and reducing the size of the solenoids because the electromagnet coils must only provide control magnetic fields, not the primary magnetic field.
This improved approach, however, continues to suffer significant drawbacks. Most particularly, high shaft speeds produce centrifugal loads that can over-stress the magnets and thrust disc rings. This has effectively limited use of the improved-design magnetic thrust bearings in applications where high shaft speeds are encountered.
Accordingly, there has been a need for a novel magnetic thrust bearing which utilizes a combination of constant flux permanent magnets and controllable electromagnets for coupling a rotatable member relative to a stationary member, in a compact and spacially efficient structure which is lightweight and obtains a high power efficiency. Additionally, there exists a need for a magnetic thrust bearing wherein magnetic efficiency of the device is optimized by minimizing air gaps between the levitated and support structures, and wherein the electromagnetic coils are not required to provide magnetomotive forces to drive magnetic flux through the permanent magnets. Moreover, a novel magnetic thrust bearing is needed which can utilize radially polarized permanent magnets, and associates such magnets with the stationary member. The present invention fulfills these needs and provides other related advantages.